This invention relates to liquid compositions suitable as inks or toners for non-impact printing. The invention is particularly related to the manufacture of such liquid inks or toners.
It has been recognised that certain properties of carrier fluids for inks or liquid developers are required for effective functioning in conventional electrostatic processes. Many of the physical requirements are mandatory, as known by those skilled in the art, but there are also other considerations, such as low toxicity, fire safety, low solvent power, low odour etc. For these reasons, isoparaffinic-hydrocarbons such as the Isopar(copyright) range manufactured by Exxon Corporation, the Shellsol(copyright) range manufactured by Shell Chemical and the Soltrol(copyright) range manufactured by Phillips Petroleum became the industry standards for liquid carriers.
In more recent times, however, increased environmental concerns have placed these non-impact liquid processes under increasing pressure to further reduce or eliminate volatile emissions.
Other carrier materials for inks or liquid developers have been investigated, and of these, silicone fluids are clearly liquids which combine all previously and currently desired properties for a modern ink and liquid toner carrier.
In general, an electrostatic ink or liquid toner for developing electrostatic images is prepared by dispersing an inorganic or organic colorant such as iron oxide, carbon black, nigrosine, phthalocyanine blue, benzidine yellow, quinacridone pink and the like into a liquid vehicle which may contain dissolved or dispersed therein synthetic or naturally occurring polymers such as acrylics and their copolymers, alkyds, rosins, rosin esters, epoxies, polyvinyl acetate, styrene-butadiene, cyclised rubber, ethylene vinyl acetate copolymers, polyethylene, etc. Additionally, to impart or enhance an electrostatic charge on such dispersed particles, additives known as charge directors or charge control agents may be included. Such materials can be metallic soaps, fatty acids, lecithin, and the like.
Silicone fluids have been used as a carrier for liquids for some time, for example, U.S. Pat. No. 3,105,821 to S. W. Johnson, and U.S. Pat. No. 3,053,688 to H. G. Greig. Both of these early patents recognised the virtues of silicone fluids, but the understanding of the functioning of liquid toners at that time was relatively empirical. More recently silicone fluids have again been recognised as desirable carrier fluids for liquid toners, as in for example U.S. Pat. No. 5,612,162 and U.S. Pat. No. 5,591,557, both to Lawson et al.
More specifically, U.S. Pat. No. 5,612,162 to Lawson et al. discloses a composition and a method of forming a liquid developer or a constituent of a liquid developer for electrostatography comprising the steps of dispersing at least one monomer in silicone fluid and polymerizing the at least one monomer to form polymer particles in the silicone fluid. A homopolymer or a copolymer may be formed and a colorant, such as a pigment or dye, and a charge control agent may be added before or after the polymerisation. The disclosure further states that the liquid developer may further include a polymerisation stabiliser which is compatible with the silicone fluid. The stabiliser may be a silicone fluid such as dimethyl polysiloxane.
It has been found that the high molecular weight, that is the 30,000 cSt and 60,000 cSt, dimethyl polysiloxane polymerisation stabilisers described in U.S. Pat. No. 5,612,162 that are compatible with silicone fluids, result in polymer dispersions that possess good suspension stability but poor fix strength, melt cohesion and adhesion properties.
If the liquid carrier component of the polymer dispersions discussed in U.S. Pat. No. 5,612,162, which is removed by oven drying or centrifugation/filtration, the polymer spheres remaining can be analysed for fix performance. In this case, the non-volatile content of this formulation will upon heating/melting followed by cooling, produce a solid that possesses minimal adhesive and cohesive properties. The solidified polymer forms a xe2x80x9cchalky likexe2x80x9d resin that can be easily abraded or broken with minimal force.
This property is obviously undesirable and when the polymer dispersion is formulated/incorporated into an ink jet ink for printing or liquid developer for electrostatography, the result is poor ink or liquid developer to substrate fix.
It is the object of this invention to provide improved polymerisation stabilisers.
It is a further objective of this invention to provide improved polymerisation stabilisers which result in the liquid ink or toner compositions exhibiting improved fix to a substrate.